用于循环优化

Question

List<String> flowers = new ArrayList<String>();

我的for循环目前看起来像这样...

for (int i = 0; i < flowers.size(); i++) {

...

}

或者我应该将其更改为如下所示的代码

int size = flowers.size();

for (int i = 0; i < size; i++) {

...

}

哪个表现更好（假设我有很多花），我猜应该是后者。

Answer 1

最好使用for-each循环 [更具可读性]for (Flower flower :flowers){&nbsp; &nbsp; //...}我已将指令转储javap用于以下代码：public void forLoop1() {&nbsp; &nbsp; List<String> lst = new ArrayList<String>();&nbsp; &nbsp; for (int i = 0; i < lst.size(); i++) {&nbsp; &nbsp; &nbsp; &nbsp; System.out.println("hi");&nbsp; &nbsp; }}public void forLoop2() {&nbsp; &nbsp; List<String> lst = new ArrayList<String>();&nbsp; &nbsp; int size = lst.size();&nbsp; &nbsp; for (int i = 0; i < size; i++) {&nbsp; &nbsp; &nbsp; &nbsp; System.out.println("hi");&nbsp; &nbsp; }}public void forLoop1();&nbsp; Code:&nbsp; &nbsp;0:&nbsp; &nbsp;new&nbsp; &nbsp; &nbsp;#2; //class java/util/ArrayList&nbsp; &nbsp;3:&nbsp; &nbsp;dup&nbsp; &nbsp;4:&nbsp; &nbsp;invokespecial&nbsp; &nbsp;#3; //Method java/util/ArrayList."<init>":()V&nbsp; &nbsp;7:&nbsp; &nbsp;astore_1&nbsp; &nbsp;8:&nbsp; &nbsp;iconst_0&nbsp; &nbsp;9:&nbsp; &nbsp;istore_2&nbsp; &nbsp;10:&nbsp; iload_2&nbsp; &nbsp;11:&nbsp; aload_1&nbsp; &nbsp;12:&nbsp; invokeinterface #4,&nbsp; 1; //InterfaceMethod java/util/List.size:()I&nbsp; &nbsp;17:&nbsp; if_icmpge&nbsp; &nbsp; &nbsp; &nbsp;34&nbsp; &nbsp;20:&nbsp; getstatic&nbsp; &nbsp; &nbsp; &nbsp;#5; //Field java/lang/System.out:Ljava/io/PrintStream;&nbsp; &nbsp;23:&nbsp; ldc&nbsp; &nbsp; &nbsp;#6; //String hi&nbsp; &nbsp;25:&nbsp; invokevirtual&nbsp; &nbsp;#7; //Method java/io/PrintStream.println:(Ljava/lang/String;)V&nbsp; &nbsp;28:&nbsp; iinc&nbsp; &nbsp; 2, 1&nbsp; &nbsp;31:&nbsp; goto&nbsp; &nbsp; 10&nbsp; &nbsp;34:&nbsp; returnpublic void forLoop2();&nbsp; Code:&nbsp; &nbsp;0:&nbsp; &nbsp;new&nbsp; &nbsp; &nbsp;#2; //class java/util/ArrayList&nbsp; &nbsp;3:&nbsp; &nbsp;dup&nbsp; &nbsp;4:&nbsp; &nbsp;invokespecial&nbsp; &nbsp;#3; //Method java/util/ArrayList."<init>":()V&nbsp; &nbsp;7:&nbsp; &nbsp;astore_1&nbsp; &nbsp;8:&nbsp; &nbsp;aload_1&nbsp; &nbsp;9:&nbsp; &nbsp;invokeinterface #4,&nbsp; 1; //InterfaceMethod java/util/List.size:()I&nbsp; &nbsp;14:&nbsp; istore_2&nbsp; &nbsp;15:&nbsp; iconst_0&nbsp; &nbsp;16:&nbsp; istore_3&nbsp; &nbsp;17:&nbsp; iload_3&nbsp; &nbsp;18:&nbsp; iload_2&nbsp; &nbsp;19:&nbsp; if_icmpge&nbsp; &nbsp; &nbsp; &nbsp;36&nbsp; &nbsp;22:&nbsp; getstatic&nbsp; &nbsp; &nbsp; &nbsp;#5; //Field java/lang/System.out:Ljava/io/PrintStream;&nbsp; &nbsp;25:&nbsp; ldc&nbsp; &nbsp; &nbsp;#6; //String hi&nbsp; &nbsp;27:&nbsp; invokevirtual&nbsp; &nbsp;#7; //Method java/io/PrintStream.println:(Ljava/lang/String;)V&nbsp; &nbsp;30:&nbsp; iinc&nbsp; &nbsp; 3, 1&nbsp; &nbsp;33:&nbsp; goto&nbsp; &nbsp; 17&nbsp; &nbsp;36:&nbsp; return它没有为我优化。Java版本“ 1.6.0_22” Java™SE运行时环境（内部版本1.6.0_22-b04）Java HotSpot（TM）客户端VM（内部版本17.1-b03，混合模式，共享）因此，如果您需要从上述两个中进行选择，请争取第二，但我个人会建议for-each。每个绩效摘自Joshua Bloch撰写的有效Java的条款46 ：版本1.5中引入的for-each循环通过完全隐藏迭代器或index变量，摆脱了混乱和出错的机会。结果成语同样适用于集合和数组：// The preferred idiom for iterating over collections and arraysfor (Element e : elements) {&nbsp; &nbsp; doSomething(e);}当您看到冒号（:)时，将其读为“ in”。因此，上面的循环读为“针对元素中的每个元素e”。请注意，即使对于数组，使用for-each循环也不会降低性能。 。实际上，在某些情况下，它可能只比普通的for循环提供一点性能优势，因为它只计算一次数组索引的限制。尽管您可以手动执行此操作（第45项），但程序员并非总是如此。

Answer 2

JVM无法优化它，因为它size()是一种方法，并且JVM无法（也不会尝试）确定size()在这种情况下总是返回相同的值。提供的size()值不会改变，第二个值的性能会更高，但是收益是如此，以至于您甚至不必考虑使用它。